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an iron-alloy structure that is a solid solution of carbon and alloying elements in α-iron. Ferrite’s crystal lattice is body-centered cubic. The solubility of carbon in the solution is 0.02–0.03 percent (by weight) at 723°C and 10–6–10–7 percent at room temperature. The solubility of the alloying elements may be very significant, even unlimited.
In most cases, alloying strengthens ferrite. Unalloyed ferrite is relatively soft and ductile, and it is strongly ferromagnetic at temperatures up to 768°–770°C. The microstructure, grain size, and substructure of ferrite depend on the conditions under which the solid solution is formed when the γ → α change occurs. With slight supercooling, polyhedral grains that are nearly equiaxed are formed; with greater supercooling and the presence of alloying elements (Cr, Mn, Ni), ferrite is formed through a martensitic transformation. The ferrite thus formed is also hardened by the transformation. The enlargement of austenite grains often leads upon cooling to the formation of Widmannstaetten ferrite, especially in cast and overheated steels. The separation of hypoeutec-toid ferrite occurs primarily at the boundaries of austenite grains. At temperatures above 1390°C, a solid solution of carbon in δ-iron is formed in iron-carbon alloys, a solution whose crystal lattice is also body-centered cubic. The solubility of carbon in δ-iron is 0.1 percent. This phase may be regarded as high-temperature ferrite.
REFERENCESBochvar, A. A. Metallovedenie, 5th ed. Moscow, 1956.
Bunin, K. P., and A. A. Baranov. Metallografiia. Moscow, 1970.
R. I. ENTIN
any of the chemical compounds that ferric oxide (Fe2O3) forms with oxides of other metals. Many ferrites combine high magnetization with semiconductor or dielectric properties, a fact explaining their widespread use as magnetic materials in radio engineering, radio electronics, and computer technology.
Ferrites contain oxygen anions (O2–), which make up the frame of the crystal lattice; Fe3+ cations, which have a radius smaller than that of the O2– anions, are positioned in the spaces between the oxygen ions. These spaces are also occupied by cations Of other metals (Mek+), which are of various radii and valences k. The coulomb (electrostatic) interaction between the cations and anions leads to a particular crystal lattice and a particular arrangement of cations within the lattice. As a result of the ordered arrangement of the Fe3+ and Mek+ cations, ferrites exhibit ferrimagnetism and typically have fairly high magnetization and high Curie points. Distinctions are made between ferrites of spinel structure, ferrites of garnet structure, orthoferrites, and hexagonal ferrites.
Ferrites of spinel structure have the general formula MeFe2O4, where Me can represent Ni2+, Co2+, Fe2+, Mn2+, Mg2+, Li1+, and Cu2+. The unit cell of these ferrites is a cube formed by eight molecules of MeOFe2O3 and consisting of 32 O2– anions, among which there are 64 tetrahedral (A) sites and 32 octahedral (B) sites partially occupied by Fe3+ and Me2+ cations (Fig. 1). Depending on the ions that occupy sites A and B and the order in which the sites are occupied, a distinction is made between straight spinel ferrites (nonmagnetic spinel ferrites) and reversed spinel ferrites (ferrimagnetic spinel ferrites). In the latter type, one-half of the Fe3+ ions are in the tetrahedral sites, with the remaining one-half, along with the Me2+ ions, in the octahedral sites. The magnetization MA, of the octahedral sublattice is greater than the magnetization MB of the tetrahedral sublattice, a difference giving rise to ferrimagnetism.
Ferrites of garnet (cubic) structure are made up of rare-earth elements R3+ (Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Sm3+, Eu3+) and yttrium Y3+ and have the general formula R3Fe5O12. The unit cell of garnet ferrites contains eight R3Fe5O12 molecules; there are 96 O2– ions, 24 R3+ ions, and 40 Fe3+ ions. These ferrites have three types of sites for the cations. Most of the Fe3+ ions occupy tetrahedral sites (d); a smaller number occupy octahedral sites (a), and the R3+ ions occupy dodecahedral sites (c). The relative magnitudes and directions of the magnetizations of the cations occupying sites d, a, and c are shown in Figure 2.
Orthoferrites are ferrites with an orthorhombic crystal structure. They are formed by rare-earth elements or yttrium according to the general formula RFeO3. Orthoferrites are isostructural with perovskite. In comparison with garnet ferrites, they have low magnetization because of their noncollinear antiferromagnetism (weak ferromagnetism); orthoferrites exhibit ferrimagnetism only at very low temperatures, of the order of a few degrees K and below.
Hexagonal ferrites have the general formula MeO(Fe2O3), where Me represents ions of Ba, Sr, or Pb. The unit cell of the crystal lattice consists of 38 O2– anions, 24 Fe3+ cations, and two Me2+ cations (Ba2+, Sr2+, or Pb2+). The cell is constructed of two spinel blocks separated by Pb2+, Ba2+, or Sr2+ ions, O2– ions, and Fe3+ ions. If ferric oxide and barium oxide are sintered with the appropriate amounts of the metals Mn, Cr, Co, Ni, and Zn, a series of ferrimagnetic oxide materials may be obtained.
Certain hexagonal ferrites possess high coercive force; these ferrites are used in the production of permanent magnets. Ferrites of garnet structure containing yttrium, certain hexagonal ferrites, and most ferrites of spinel structure are used as soft-magnetic materials.
With the introduction of admixtures and the creation of a nonstoichiometric composition (with variations in composition with respect to both the cations and oxygen), the electrical resistance of ferrites is altered within a wide range. Ferrites are not used in semiconductor technology owing to the low mobility of the current carriers. The synthesis of polycrystalline ferrites is carried out through the methods of ceramic technology. Items of the required form are molded from a mixture of the starting oxides, which are then sintered at temperatures between 900°C and 1500°C in the air or in a special gas medium.
Single crystals of ferrites are grown by the Czochralski and Verneuil methods.
REFERENCESRabkin, L. I., S. A. Soskin, and B. Sh. Epshtein. Ferrity: Stroenie, svoistva, tekhnologiia proizvodstva. Leningrad, 1968.
Smit, J., and H. Wijn. Ferrity. Moscow, 1962. (Translated from English.)
Gurevich, A. G. Magnitnyi rezonans v ferritakh i antiferro-magnetikakh. Moscow, 1973.
K. P. BELOV